This is the point of White's play.
12... c5b4 Necessary. 13. g5f7 A new move. I wonder whether Harikrishna had prepared this when analyzing his blitz game against Ganguly. In this game, he spent some time trying to remember the lines. 13... e8f7 Warmerdam evidently was ready for the knight sac and kept blitzing his moves. 14. e4g5  14... f7f6 